This invention relates to an apparatus for drying dielectric oils and more particularly to an apparatus for removing water from dielectric oil used in electrical power transformers.
A key component in delivering electrical energy to the consumer is the dielectric oil-filled power transformer. Electricity is typically generated at some location distant from the point of consumption. To deliver it economically, the voltage from the generator is increased to higher levels by step-up transformers for transmission to a number of substations where step-down transformers reduce the voltage for industrial and municipal use. Electricity is distributed at even lower voltages for domestic use. Transformers are critical links in the delivery of power from the source to the consumer in an economical and reliable fashion. Small power transformers are typically mounted on utility poles positioned along streets, at malls, and industrial sites, whereas moderate to large size units are located at substations and generating stations.
Transformers are primarily composed of conductors that may be copper or aluminum, and insulating materials that are typically mineral oil and paper. The mineral oil serves two functions: 1) as a coolant to minimize overheating of the transformer, and 2) as a dielectric providing sufficient strength to prevent dielectric breakdown between conducting parts at different electrical potentials and also between the energized parts and the grounded tank. The paper serves as an electrical insulator and provides mechanical support for the electrical conductor packages. A large transformer may contain 30,000 gal (113 550 L) of dielectric oil and 30,000 lbs (13 610 kg) of paper insulation.
Transformer insulating materials such as mineral oil and paper must be properly dried during the manufacturing process. A certain degree of dryness must then be maintained during the service life of the unit.
In order for the transformer to operate properly, the insulating materials must be dried to a proper level. The dielectric breakdown voltage of oil is reduced with increasing water content. As dielectric breakdown voltage is also affected by the presence of particles, it can not be used to predict the water content.
Oil is typically dried to less than 10 ppm water by weight and can increase to 20-30 ppm in use depending upon the voltage class of the equipment. A worst case situation arises when the water content, even at low parts per million, becomes greater than that required for saturation of the oil, and free water is formed. This results in drastically-reduced dielectric breakdown strength of the oil with the risk of failure of the transformer greatly increased.
The paper insulation must also be free of water to achieve good dielectric properties and enhance resistance to degradation. The concentration of water is usually between 0.1 to 0.5% by weight. The quantities of water in the paper insulation is much greater than that in the oil. When oil and paper are together, water will partition between them primarily based upon temperature.
As more demand or load is placed on a transformer, the insulating materials increase in temperature. The elevation in temperature forces water from the paper insulation into the dielectric oil. As demand is reduced and the transformer cools, the water migrates from the oil to the paper.
The paper must remain in a relatively dry state to prevent excessive water from being driven into the oil at hotter temperatures. It is actually during the cooling cycle that the water will exceed saturation in the oil, creating a problem.
The key to preventing failures in transformers from excessive water is proper monitoring.
A crucial step in obtaining reliable water measurements is acquiring a representative oil sample.
Transformers are tested at the factory to ensure drying procedures are adequate. Testing of dew point, vapor pressure, and electrical properties are used to assess the water content of the paper insulation before the transformer is filled with oil. Measurement of water in the oil is performed during and after filling of the transformer. In some cases the transformer may be shipped with dry air or nitrogen under slight positive pressure and then oil filled in the field.
Once the transformer is filled with oil, it is extremely difficult and almost impossible, for practical purposes, to obtain paper samples. Therefore, evaluation of the dryness of the paper insulation is performed by external overall measurement of the electrical properties of the transformer winding insulations that are influenced by water, or by taking samples of oil, recording the oil temperature, and using appropriate correlation curves to determine water content of the paper and dielectric oil. The latter can be effectively performed only if the transformer has been maintained at a steady warm temperature for many days to allow the system to come to a steady state.
Oil from transformers in the field should be checked for water content to make sure the manufacturer properly dried the paper insulation, to see if a leak has occurred, and because insulation degradation results in water formation over long periods of time. Careful monitoring can help prevent the conditions that will result in the formation of free water above the saturation level in oil and subsequent failure of a transformer. A margin of safety is provided by maintaining the insulation dry enough so that under all operating conditions, the water in oil is at less than 50% saturation.
Presently, in order to effect the desired dehydration of the dielectric oil in an energized operating transformer involves a process wherein the transformer containing wet dielectric oil must be taken out of service. The dielectric oil is removed from the transformer and is serviced by heating the oil above about 200xc2x0 F. under high vacuum to remove built-up dissolved water. This is generally time consuming and accomplished by circulating the oil to be treated through a heating and evacuating system mounted on a mobile service truck. For large transformers, the drying process may require several attendants overseeing the removal, working in twelve hour shifts for a period of one week.
It is an object of the present invention to produce an apparatus for removing water from the dielectric oil in an energized electrical power transformer while the transformer is in service.
Another object of the invention is to produce an apparatus for removing water from the dielectric oil in an energized electrical power transformer including the vacuum pump for militating against air entering the transformer during the period the water is being removed from the dielectric oil.
Still another object of the invention is to produce an apparatus for removing water from the dielectric oil of an energized transformer including a monitor for sensing the water content of the dielectric oil before and after the oil is caused to be passed through the treating filter cartridges in the evacuated filter housing or vessel.
The above, as well as other objects of the invention, may be readily achieved by an apparatus for removing water from the dielectric oil in an electrical power transformer comprising: a pump for circulating the dielectric oil from a transformer and returning the same to the transformer; a filter in fluid communication with the pump for removing water from the dielectric oil; a vacuum pump in fluid communication with the filter for preventing air from being introduced into the transformer; and a sensor for monitoring the dryness of the oil after circulation through the filter.